The head of a conventional gamma camera comprises a collimator for receiving radiation stimuli from a radiation field, a scintillation crystal which produces light events at spatial locations corresponding to the locations at which stimuli passing through the collimator interact with the crystal, and an optical system for transmitting light produced by the light events to an array of photomultipliers. Each photomultiplier produces an output signal that depends on the spatial location of the event in the crystal relative to the photomultiplier; and these output signals are applied to signal processing equipment which calculates the x, y coordinates of an event based on assigning different weights to the output signals of the photomultipliers according to their location in the array.
An event can be recorded by using the calculated coordinates to control the deflection of a CRT whose intensity control gates on the beam for a predetermined short period of time if the total energy of the light event lies within a predetermined energy window as determined by a single channel analyzer. In such case, a brief flash of light appears on the screen at the calculated coordinates of each event; and a film exposed to light from the screen for a predetermined period of time will provide a record of the intensity distribution of the radiation field by recording the light events as gray-scale variations in the film.
In other systems, events can be recorded in a digital memory having a register associated with each picture element of the field of view of the camera. In this case, the occurrence of an event at a given calculated x, y coordinate location is recorded by indexing the memory register at the address corresponding to the coordinates of the light event. After accumulating counts for a given time interval, an image of the radiation field is produced by displaying the contents of the memory registers on a CRT, the brightness of a picture element of the display being directly related to the contents of the corresponding memory register.
Whichever approach is used to produce an image of a radiation field, the quality of this image is determined by the resolving power of the camera and its sensitivity or homogeneity. The resolving power of a camera is its ability to distinguish between stimuli originating in adjacent locations in a radiation field, and is determined mainly by the optical system of the camera. The sensitivity of a gamma camera, on the other hand, is concerned with the uniformity of the response of a camera to a uniform distribution of stimuli over the active area of the crystal. The present invention is concerned with improving the sensitivity of a gamma camera rather than improving its resolution. As is well known in the art, an image produced by flooding an uncompensated gamma camera with a uniform radiation field will produce a variegated pattern whose brighter regions closely match the photomultiplier array. Particularly in a nuclear medicine environment, this inhomogeneous response to a uniform radiation field, if uncompensated, will reduce the accuracy of fidelity of an image of an unknown non-uniform field such as is produced by a human patient into whom a radioactive pharmaceutical is injected.
An important contribution to the inhomogeneity or variation in sensitivity of a gamma camera is the dislocation or distortion of the image due to deviations from linearity of the responses of the photomultipliers of the camera head. The response of an individual photomultiplier across its photosensitive surface is non-linear, and in fact, is quasi-Gaussian in nature; and it is conventional to manipulate the outputs of the photomultipliers by weighting them such that, as a group, the photomultipliers have a substantially linear response. Nevertheless, the overall response of the photomultipliers is not exactly linear with the result that the calculated coordinates of an event will be displaced from the actual coordinates of an event. Furthermore, the displacement of the calculated position from the true position of an event is spatially dependent.
Dislocation or distortion of a gamma camera image can be compensated for by following the technique disclosed in U.S. Pat. No. 3,745,345 (whose disclosure is hereby incorporated by reference). In such technique, a given camera is calibrated before being used for imaging a patient by interposing a pierced shield between the crystal and a uniform radiation field. The locations of the holes in the shield are precisely known so that the coordinates of a given hole can be compared with the first moment of the distribution of counts associated with the given hole as calculated by the signal processing equipment of the camera. The process is repeated after shifting the shield and until a 2-dimensional dislocation map of the desired density of points over the entire crystal is created, each entry in the map being the correction factor which must be applied to the calculated coordinates of an event (from which a distorted image of the radiation field can be constructed) in order to relocate the events and provide a less distorted image. The dislocation correction factors are thus predetermined in accordance with deviations from linearity of the photomultipliers to stimuli that interact with the head. Once such a map of dislocation factors is obtained, the calibration mode is terminated and the camera is used for imaging a patient for a period of time during which it is assumed that the dislocation process remains constant.
The dislocation map can be used to correct the image on an event-by-event basis (i.e., looking up the correction factor for each event as it occurs and relocating it for storage at its corrected location rather than its distorted location), or corrections can be done after a complete, albeit distorted, image is obtained. In the latter case, correction factors are applied to the counts in each picture element of a digital memory map of the distorted image according to the contents of the dislocation map.
Another significant contribution to the inhomogeneity of a gamma camera arises from the spatial dependence of the so called Z-signal of a gamma camera. As discussed in U.S. Pat. No. 4,095,108 (whose disclosure is hereby incorporated by a reference), the amount of light received by the photosensitive surfaces of all the photomultipliers of an array in a camera due to a given interaction of a radiation stimulus of fixed energy with the crystal, is a function of the position of the light event in the crystal. The spatial dependence of the total energy of a light event can be taken into account by a so-called Z-signal correction map which tabulates the spatial dependence of deviations in total energy of events due to inhomogeniety of the optical system which includes the crystal light guide and photomultipliers. If the total energy of the event lies inside the energy window determined by the Z-correction map at the calculated coordinates of the event, then the event is "counted" by indexing a counter in a digital memory at an address corresponding to the calculated coordinates of the event.
As in the case of distortion correction, Z-signal correction improves image quality over a non-compensated gamma camera; and the image quality is greatly improved if both dislocation and Z-signal compensations are utilized.
Neither of these techniques for improving image quality of a gamma camera, however, takes into account the inherent mechanical imperfections of a camera such as the non-uniform stopping power of a crystal, or differences in the geometry of the many openings in the collimator. The stopping power of a crystal or of the collimator relates to the ability of a target receiving stimuli to interact with the stimuli. The stopping power is generally expressed in terms of the ratio of the number of particles that interact with a target to the number of particles incident on the target. As indicated above, both the crystal and the collimator are likely to have spatially dependent stopping powers, no matter how well they are constructed. Thus, correction for distortion or Z-signal inhomogeneity will not eliminate image distortions due to inherent mechanical imperfections in the camera components.
In an effort to provide compensation simultaneously for all three factors affecting the inhomogeneity of a gamma camera, it is conventional to provide what is termed a "flood correction" in which a camera is flooded with a uniform radiation field and the resulting image is used to construct a "flood map". Because the input stimuli at each elemental area of the crystal is known to be constant, deviation in the brightness of an elemental area of the image from the average brightness of the entire image permits a correction factor to be calculated which can be applied to a map produced by a gamma camera in normal usage. The correction factors over the entire crystal can be stored in a digital map and called up to correct the raw data obtained by the camera. For example, if a given picture element is say 10% darker than the average brightness of the image obtained under flood conditions, the number of counts recorded in this picture element under imaging conditions would be increased by 10%. While this technique provides a simple approach to image correction, and often provides improved results, it is not a sound approach to image correction.
The inherent deficiency in this technique is illustrated by assuming the given picture element images an element of a radiation field where there is actually a low level of stimuli emitted in comparison to an adjacent region. With the conventional flood correction, the counts accumulated in this picture element would be increased by 10% thus concealing the actual situation. Such a display of image portions that are not actually present in the object being imaged is termed an artifact; and the presence of artifacts in gamma camera images is highly undesirable for obvious reasons.
Therefore, an object of the present invention to provide a new and improved gamma camera correction circuit and a method for using the same wherein the resulting image is improved over the image obtained with conventional correction techniques.